I. Field of the Invention
The present invention relates to data communications. More particularly, the present invention relates to method and apparatus for efficiently demodulating signals that have been processed and transmitted in a diversity mode.
II. Description of the Related Art
In a typical digital communications system, data is processed, modulated, and conditioned at a transmitter unit to generate a modulated signal that is then transmitted to one or more receiver units. The data processing may include, for example, formatting the data into a particular frame format, encoding the formatted data to provide error detection and/or correction at the receiver unit, channelizing (i.e., covering) the coded data, and spreading the channelized data over the system bandwidth. The data processing is typically defined by the system or standard being implemented.
At the receiver unit, the transmitted signal is received, conditioned, demodulated, and digitally processed to recover the transmitted data. The processing at the receiver unit is complementary to that performed at the transmitter unit and may include, for example, despreading the received samples, decovering the despread samples to generate decovered symbols, and decoding the decovered symbols.
In some communications systems, data is processed and redundantly transmitted over two (or possibly more) antennas to provide transmit diversity. The processing may include, for example, covering the data for each antenna with a particular channelization code (e.g., a particular Walsh symbol). In some systems, the data for one or more antennas may also be reordered prior to the channelization. Due to multipath and other phenomena, the transmitted signals may experience different path conditions and may arrive at the receiver unit at different times. If the transmit antennas are spaced sufficiently far apart, then the received signals from the antennas tend to fade independently. Each transmitted signal may also reach the receiver unit via multiple signal paths. The receiver unit is then required to receive, track, and process one or more instances of each transmitted signal, and to combine the results from the processed signal instances to recover the transmitted data. On the downlink, the processing typically includes tracking a pilot that has been transmitted along with the data, and using the recovered pilot to demodulate data samples.
The signal processing (e.g., demodulation) to process multiple transmitted signals, and multiple instances of such signals, can be complicated. Moreover, transmit diversity is typically provided on the downlink, and user terminals are required to support such a mode. The user terminals are typically more impacted by complexity and costs considerations. Therefore, techniques that can be used to efficiently demodulate signals that have been processed and transmitted in a diversity mode are highly desirable.
The present invention provides demodulator architectures, demodulators, and receiver units for processing signals that have been processed and transmitted in a transmit diversity mode. When operating in the transmit diversity mode, data symbols are typically covered with a channelization code (e.g., a Walsh symbol) having a length (2T) that is twice the length (T) of the channelization code used to cover the data symbols in the non-transmit diversity mode. The demodulator architectures of the invention exploit this property and perform partial processing (e.g., despreading, decovering, pilot demodulation, or a combination thereof) on each fraction of a channelization symbol period of 2T. The processed xe2x80x9cpartial-symbolsxe2x80x9d are then appropriately combined to generate the demodulated symbols. By performing partial processing on each fraction (e.g., each half) of the symbol period of 2T, computational complexity and costs can be reduced and performance may be improved. For example, with the present invention, the pilot demodulation in each assigned correlator (i.e., finger) can be performed based only on pilot estimates generated by that correlator, whereas conventional techniques may require pilots from multiple correlators. Other advantages are described below.
An embodiment of the invention provides a demodulator for processing a received signal in a wireless communications system. The demodulator includes a number of correlators coupled to a combiner. Each correlator typically receives and despreads input samples with a respective despreading sequence to provide despread samples. The input samples are generated from the received signal. Each correlator then decovers the despread samples to provide decovered xe2x80x9cpartial-symbolsxe2x80x9d and further demodulates the decovered partial-symbols with pilot estimates to generate correlated symbols. The decovering is performed with a channelization symbol (e.g., a Walsh symbol) having a length (e.g., T) that is a fraction (e.g., half) the length 2T of the channelization symbol used to cover the data symbols in the received signal. The combiner receives and selectively combines correlated symbols from the assigned correlators to provide demodulated symbols.
In the transmit diversity mode of a CDMA-2000 or W-CDMA standard (which are identified below), the received signal includes a pair of signals transmitted from a pair of antennas. One or more correlators can then be assigned to process at one or more instances of each transmitted signal. Each assigned correlator processes the received signal to recover pilot estimates corresponding to the signal instance being processed. The pilot estimates are then used within the assigned correlator to demodulate the decovered partial-symbols.
A specific embodiment of the invention provides a demodulator that includes a number of correlators coupled to a combiner. Each correlator typically includes a despreader, a decover element, a complex multiplier, and a switch coupled in series. The despreader receives and despreads input samples with a particular despreading sequence to provide despread samples, and the decover element decovers the despread samples to provide pairs of decovered half-symbols. The decovering is performed with a Walsh symbol W having a length (T) that is half the length (2T) of a Walsh symbol WSTS used to cover the data in the received signal. (Space-Time Spreading (STS) is a transmit diversity mode defined by the CDMA-2000 standard.) One pair of decovered half-symbols is provided for each Walsh symbol period of 2T. The complex multiplier then demodulates the decovered half-symbols with a pilot recovered by the correlator to provide demodulated half-symbols.
The switch provides a first combination of decovered half-symbols for each Walsh symbol period of 2T in a first (e.g., even) symbol stream and a second combination of decovered half-symbols for each Walsh symbol period of 2T in a second (e.g., odd) symbol stream. The combiner combines the first symbol streams from the correlators to provide a first (even) output symbol stream, and further combines the second symbol streams from the correlators to provide a second (odd) output symbol stream.
In one design of this specific embodiment, the multiplier in each correlator performs a dot product and a cross product between the decovered half-symbols and the pilot to provide xe2x80x9cdotxe2x80x9d symbols and xe2x80x9ccrossxe2x80x9d symbols, respectively. The combiner can then be designed to selectively combine the dot and cross symbols for each Walsh symbol period of 2T to provide the demodulated symbols for the first and second output symbol streams.
Another specific embodiment of the invention provides a demodulator that also includes a number of correlators coupled to a combiner. Each correlator typically includes a despreader, a decover element, first and second summers, and first and second complex multipliers. The despreader receives and despreads input samples with a particular despreading sequence to provide despread samples, and the decover element decovers the despread samples to provide pairs of decovered half-symbols. Again, the decovering is performed with a Walsh symbol W having a length (T) that is half the length (2T) of a Walsh symbol WSTS used to cover data symbols in the received signal, and one pair of decovered half-symbols is generated for each Walsh symbol period of 2T.
Each correlator typically further includes a switch coupled to the decover element. The switch provides decovered half-symbols corresponding to the first half of the Walsh symbol period of 2T to a first output and decovered half-symbols corresponding to the second half of the Walsh symbol period of 2T to a second output. Each summer then operatively couples to the outputs of the switch and combines each pair of decovered half-symbols in a particular manner to provide a decovered symbol. Each multiplier then demodulates the decovered symbols from a respective summer with a respective pilot to provide a respective symbol stream.
The combiner receives the first and second symbol streams from the first and second multipliers, respectively, of each assigned correlator, combines the first symbol streams from all assigned correlators to provide a first output symbol stream, and further combines the second symbol streams from all assigned correlators to provide a second output symbol stream.
Another embodiment of the invention provides a method for processing a received signal in a wireless communications system. The received signal can include a pair of signals transmitted from a pair of antennas. In accordance with the method, input samples are generated from the received signal. At least one signal instance of each transmitted signal is then processed to provide correlated symbols. The processing for each signal instance typically includes despreading the input samples with a particular despreading sequence associated with the signal instance being processed to provide despread samples, decovering the despread samples to generate decovered partial-symbols (e.g., half-symbols), and demodulating the decovered partial-symbols with pilot estimates to generate the correlated symbols for the signal instance. Again, the decovering is performed with a Walsh symbol W having a length (e.g., T) that is a fraction of (e.g., half) the length (2T) of a Walsh symbol WSTS used to cover the data in the received signal. The correlated symbols for all signal instances being processed are then selectively combined to provide demodulated symbols.
The invention further provides other demodulator architectures, correlators, demodulators, receiver units, and methods to process signals that have been processed and transmitted in a transmit diversity mode